1. Field of the Invention
The present invention relates to curable organopolysiloxane compositions which are both photo-curable and room-temperature-curable.
2. Description of the Prior Art
It is well known that organopolysiloxanes, when heated in the presence of an organic peroxide, become elastic silicone rubber products which are excellent in heat resistance, cold resistance, weatherability and electrical properties. Among such organopolysiloxanes are those which are curable by irradiation with light in the presence of a photoinitiator.
For instance, Japanese Patent Publication (KOKOKU) 52-40334 (1977) and Japanese Pre-examination Patent Publication (KOKAI) 60-104158 (1985) disclose ultraviolet (UV)-curable type organopolysiloxane compositions which comprise a vinyl group-containing polysiloxane and a mercapto group-containing polysiloxane and which give cured products through photo radical addition reactions.
Also, Japanese Patent Publication (KOKOKU) 53- 36515 (1978) and Japanese Pre-examination Patent Publication (KOKAI) 60-215009 (1985) disclose photo-curable type organopolysiloxane compositions which comprise an acryloxy group (--OCOCH.dbd.CH.sub.2) containing polysiloxane and a sensitizer and which give cured products when irradiated with light.
However, the above-mentioned compositions containing a vinyl group-containing polysiloxane and a mercapto group-containing polysiloxane have the problems of odor and metal-corroding property both ascribable to the mercapto group, and are thereby limited in application.
On the other hand, the above-mentioned compositions comprising an acryloxy group-containing polysiloxane and a sensitizer have the problem that, in order to obtain rubber-like elastic products, the acryloxy group-containing polysiloxane must be or comprise a linear polysiloxane with a high molecular weight. When such polysiloxane is used, the amount of acryloxy groups located at terminal ends is very small on a relative basis, leading to a lowered curability. In addition, these compositions have the drawback that their surface portions exposed to air are hardly cured, because of inhibition by oxygen. Of this type of compositions, therefore, only those containing a polysiloxane with a relatively high acryloxy group content and intended to form resinous cured products have been put to practical use; those for giving rubber-like elastic products have not yet been put to practical use.
Besides, although the photo-curable organopolysiloxane compositions as above mentioned are capable of speedy curing by irradiation with UV rays and are advantageous over conventional condensation type, heat-curable type and platinum-catalyzed addition reaction type organopolysiloxane compositions with respect to workability, the elastic silicone rubber products resulting from the photo-curable compositions are inferior in tensile strength. Further, the photo-curable compositions lack in storage stability. Thus, there are restrictions on the application of the photo-curable organopolysiloxane compositions.